The subject of the invention is a process for the reduction of the particle sizes of coarse grain, crystalline, zeolitic sodium aluminosilicates of the NaA type, which are obtained in an aqueous alkaline suspension by the crystallization of a suspension reaction mixture formed from sodium aluminate and sodium silicate solutions.
The so-called zeolites form a mineral class of alkali metal aluminosilicates that contain water of crystallization and have a defined structure of pores and hollow spaces in their aluminosilicate lattice. Synthetic zeolites have gained increasingly technological significance and are used, for example, as cation exchange agents mainly for the softening of water, as catalyst carriers for chemical processes, as drying, parting or sorption agents for solvents and gases ("molecular sieves"), as well as heterogeneous inorganic builders in washing and cleaning agents. Depending on the intended purpose, structurally differing zeolite types and also different degrees of dryness and purity are needed. Such zeolites generally are first prepared in their sodium form and, if desired, subsequently converted into other forms by cation exchange.
With respect to the intended purposes mentioned above, especially the zeolitic sodium aluminosilicate of the NaA type has become technically important. The chemical composition of this type of zeolite approximately corresponds to the empirical formula: EQU 1.+-.0.2Na.sub.2 O.1Al.sub.2 O.sub.3.2.+-.0.5SiO.sub.2.0 to 6H.sub.2 O.
The characteristic x-ray diffraction diagram of the zeolite NaA is described, for example, in U.S. Pat. No. 2,882,243.
A very finely powdered zeolite with as narrow a particle size distribution as possible and an average particle size of less than 10 .mu.m is generally preferred for most technical applications. Furthermore, especially when the zeolite NaA is used in washing and cleaning agents, its content of particles with a particle size above 50 .mu.m--called "grit" in the following text--should not amount to more than 0.2 percent by weight, or preferably be below this limit, and its cation exchange capacity should be relatively high.
Zeolitic sodium aluminosilicates generally are prepared by discontinuous or also continuous mixing of an aqueous sodium aluminate solution with an aqueous sodium silicate solution in the presence of excess sodium hydroxide solution at an elevated temperature. The x-ray-amorphous sodium aluminosilicates formed in a primary reaction usually are converted into the respective crystalline zeolite forms by subsequent heating of the reaction mixture suspension for a certain length of time. The formation of crystalline products requires a period of time from a few minutes to several days, depending on the molar rations of the reaction partners in the starting batch and the regulation of the temperature. This crystallization process is carried out mainly under atmospheric pressure and at temperatures in the range from 70.degree. to 100.degree. C. for the preparation of the zeolite NaA. This procedure, together with a respective composition of the reaction mixture, generally results in highly crystalline zeolites of the type NaA with a great cation exchange capacity, which are first obtained in the form of an aqueous alkaline suspension.
Because of the desired small particle sizes of the formed zeolite NaA, until now it has been attempted to keep the proportion of grit as low as possible by the choice of certain suitable reaction parameters for the zeolite synthesis, such as temperature, period of mixing of the reaction partners, composition of the reaction mixture and other such measures. A particular proposal made in this connection was to allow strong shear forces to act on the reaction mixture during the mixing of the reaction partners and, if desired, also during the crystallization step, to counteract the formation of coarse-grained products in this manner.
To reduce the particle size, zeolite crystals, which were isolated from the mother liquor and dried, were also milled in a ball mill and the obtained fractions were separated in a centrifugal sifter. Most of the described steps involve considerable technical work, however, and may also in addition result in a lower space-time yield of the entire process because of the special, and thus in some respects restricted, procedural course. Despite optimal process conditions, the obtained grit contents usually are within the range from 0.05 to 0.3 percent by weight, based on the total yield of zeolite NaA.